The type of adjustment to environmental conditions which does not result in any recognizable alteration in the structure of the plant, but simply permits it to grow under new conditions, manifests itself in many ways. These adjustments are usually associated with differences in temperature during the growing season, and for this reason, most such examples of adaptation have been studied in connection with possible temperature reactions upon the growing organism.
However, recent investigations seem to point strongly to the conclusion that the amount of _light_ rather than the _temperature_ of the new surroundings is the most important influence in determining the physiological processes known as the "acclimatization" of plants. For example, a very elaborate series of investigations has shown that the flowering stage in the development of plants is determined by the length of the daylight period per day, irrespective of the actual amount of vegetative growth which the plant has made. Thus, tobacco plants, which during a period of long days grow to the height of 8 or 10 feet before blossoming, if grown at the same temperature in periods of short days (or if kept in the dark during a portion of the longer days) will blossom when less than 3 feet in height and when the total mass of vegetative material which has been produced is less than one-third of that of the "gigantic" plants of the same variety grown with longer periods of illumination per day. This same principle has been found to hold good for many widely different types of plants. In some species, however, flowering is favored by long days, and vegetative growth by short daylight illumination. But in all species which have been studied, there seems to be a direct relation between the length of day, or the total illumination per day, and the normal or abnormal functioning of the plant. It is apparent that at least the physiological function of sexual reproduction (flowering and seed-production) is determined by the length of daylight illumination. The duration of daylight per day which is necessary to induce the blossoming of the plants varies for different species, but it is constant for individuals of the same species. This adaptation of stage of growth to duration of daily illumination must, therefore, be an evolutionary character of the species.